A prior art cathode-ray tube comprises an evacuated casing which accommodates an electron optic system, a laser screen and a getter pump (Ulasyuk, V. N. Kvantoskopy [Laser Screen Cathode-Ray Tubes] (in Russian). Moscow. "Radio i Svyaz" Publishing House. 1988. p. 105). This is a sealed-off cathode-ray tube, and its electromagnetic focusing lens and a deflecting system are positioned outside the casing. The laser screen is cooled to a liquefied gas temperature, and a high voltage of 50 to 65 kV is applied to a cathode. A signal for modulating electron beam current is applied to an electrode near the cathode and is decoupled by means of an optical coupler. Hairpin cathodes are not used in such cathode-ray tubes because it is not possible to adjust their position. This cathode-ray tube is deficient because of a poor resolution on the laser screen. This disadvantage stems from a large diameter of an electron spot on the laser screen and a narrow modulation frequency band.
Another prior art cathode-ray tube has an electron gun with a tungsten hairpin cathode, a modulator comprising a pair of deflecting plates, a diaphragm and two pairs of correcting plates of a matched length, an electromagnetic focusing lens, a deflecting system, and a laser screen mounted in a toroidal cryostat (Nasibov, A. S. et al. Laser Screen Cathode-Ray Tube. (in Russian). J. Kvantovaya Elektronika. Moscow. 1974. Vol. 1. No. 3. pp. 538-540). This cathode-ray tube can be disassembled. It has a small-diameter electron spot on the laser screen. High voltage is applied to the cathode, and the modulator is positioned downstream an anode which is at a zero potential. One of disadvantages of this cathode-ray tube resides in the fact that the electron beam cannot be adjusted in operation. This adjustment of electron beam is necessary because the position of the point of the hairpin cathode with respect to other electrodes of the cathode-ray tube fluctuates during operation. This cathode drift results in a change in the time during which electron beam passes through the modulator diaphragm. The modulation characteristic of the cathode-ray tube also varies. This instability cannot be completely eliminated by using other types of hairpin cathodes and different cathode materials because the cathode drift is inevitable. Another disadvantage resides in a comparatively high modulator cut-off voltage (200 to 300 V) and high thermal load on the diaphragm. This requires the use of sophisticated and more expensive video amplifiers and results in a shorter service life of the modulator and the tube as a whole.